One solution:

1. Open "Organize Bookmarks"
2. Click on "Unsorted bookmarks" on the left side
3. Click on the "Organize" tab at the top.

You should now be able to add a new bookmark or new folder

To add a new separator:

Steps 1-2 above, then:

3. Click anywhere in the upper right box
4. Click on the "Organize" tab

You should now be able to select new separator

Hope this helps.

You can create new folders and new bookmarks in the three main folders ("Bookmarks Toolbar","Bookmarks Menu","Unsorted Bookmarks") or their sub folders, but you can not create a new folder at the root level ("All Bookmarks").

In the Bookmarks Manager you can also create a new folder via the Organize menu.

In the Bookmarks sidebar (Ctrl+B) you can do that via the right-click context menu.

If you want the new bookmark or folder to appear on the Bookmarks Toolbar then create that new bookmark or folder in the "Bookmarks Toolbar" folder.
Otherwise create the new bookmark or folder in the "Bookmarks Menu" folder or in a sub folder in that folder.